For a long time, isotactic polymers have been of interest as plastic materials for manufacturing articles of relatively good deformation resistance, such as sheathings of household appliances. In general, such isotactic polymers with propylene as monomer are of highly crystalline nature and, therefore, are relatively hard with little or no impact resistance such that they are useful only in applications in which those characteristics are desirable.
Most recently, various attempts have been made, aiming at preparing also polypropylene with elastic characteristics. EP 0 707 016 A1 specifies a catalyst composition and a process for preparing polyolefins. The catalysts specified in EP 0 707 016 A1 are, in substance, made up of a metallocene compound having an indene ring and a fluorene ring which are bridged via C, Si or Ge. In case of the metallocene compound, it is essential that, in the indene ring system, at least the residue denoted with R4 not be hydrogen. When that residue R4 is hydrogen, the effects will not be attained. The polymers specified in EP 0 707 016 A1 prepared with metallocenes, especially the polypropylene prepared with those metallocenes, however, have shown unsatisfactory characteristics in regard of the elastic behavior.